Head Banging weird Mountain Lion Network Issues

Okay, so the recent Mountain Lion Install went fairly smoothly with only a nfew hiccups. That was until I tried to update my blog post …

At first, everything worked as expected, but when I tried to update a longer post, the POST just timed out. I wan't too upset, it's my own bloging engine, so I just assumed my code would at fault. Later, I noticed a few other wweb pages weren't working either, and not samll sites, big ones, like facebook.com and apple.com, even apples disscussions.apple.com wouldn't load, and Dropbox had stopped syncing. Something was very wrong. Anyway, I was at work and couldn't spend all day fixing my MacBook, so I just switched to my iMac, still running Lion, everything was fine. Obviously a bug in the initial version of the OS.

Later at home, I fired up my MacBook, to try and resolve the problem. Everything worked fine. All of my problems at work just didn't manifest on my home network.

Next day, back at work, and the problems were all back, execectly the same. Now I had a very Mountain Lion specific bug only on the network at work, without any clue as to what was going on.

The Solution

A bit searching later, I stumbled across this post by Matt, from 2011, about imporving network performance by changing the Hardware MTU setting. His post does a great job explaing what the MTU does and how it effects netowrk traffic. So, I gave it a go to see if it would help.

The basic test to try is to send a TCP ping packet to a server, in my case the aunty beeb. The import part is setting the size, in bytes, of the packet and not allowing the payload to be spread of mutliple packets. This gives you your network MTU.

ping -c 2 -D -s 1464 bbc.co.uk

If the command Timesout, try lowering the -s. If the command pings back correctly, try upping the -s flag. The tick is to use the figure + 28 in your network settings.

Now, even though Mountain Lion shoudl be able to autodetect this, apparenetly its not as good as Lion was. By changing my MTU (in Preferences->Network->Advanced->Hardware), from 1500, to 1490, everything works fine. In fact, its better than fine, it's now pretty quick!

Now, i'm hoping this is only a temporary fix, and that the first service update to Mountain Lion fixes the autodetection, as i really dont want to have to check the MTU of every network I join!